BODIPY dyads and triads: synthesis, optical, electrochemical and transistor properties
© The Author(s) 2018
Received: 2 December 2017
Accepted: 4 May 2018
Published: 11 May 2018
Organic semiconductors are crucial component in organic photovoltaics since they served as both light harvesting unit and charge transporting material that involved in energy conversion process. To effectively convert solar energy into electrical current, the organic semiconductors should have broad and intense absorption to harvest photon flux form the solar spectrum, proper HOMO and LUMO energy levels and sufficient charge carrier mobility to facilitate charge separation process [1, 2]. Typically, organic semiconductors consist of π-conjugated system which can be either small molecules or polymer based aromatic compounds. Polymer based semiconductors offer broader absorption, low cost deposition processing in small and large area [3–5]. However, they are polydisperse and tended to have batch-to-batch variation, higher molecular disorder and impurity from the end groups [6, 7]. In contrast, small π-conjugated molecules provide benefit on high purity with defined chemical structures, precise molecular weight and synthetically reproducible [6, 7]. These make small molecules gaining more attention to utilize in organic photovoltaics.
Recently, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacen or boron dipyrromethene (BODIPY) has been explored for optoelectronic applications [8–10]. BODIPY is attractive heteroatom building block for organic semiconductor because it possesses excellent absorption properties with high molar absorptivity, high quantum yield and high photo-bleaching life time [11, 12]. The BODIPY core has three locations, the meso-position, the pyrrolic positions and the boron atom position, in which π-conjugation substituents can be attached [13, 14]. The effect of BODIPY structures on photophysical properties has been intensively explored [10, 13], while fundamental understanding on relationship between structures and charge transport property is less investigated. Generally, BODIPY based small molecules have been designed using symmetrical D–A–D and A–D–A triads [15, 16], where donor (D) is an electron rich functionality and acceptor (A) is a BODIPY unit. For instances, Krishnamoothy et al.  studied the effect of alkyl side chains at the meso-position of triphenylamine-BODIPY-triphenylamine triads on the charge transport properties. The hole mobilities of those BODIPY were found in the range of 10−5–10−7 cm2/V s. Recently, Thayumanavan et al.  developed the A–D–A structures where various donor moieties were incorporated at the meso-position of the BODIPY. They found that insertion of cyclopentadithiophene as the donor unit provided the highest electron mobility in the range of 10−5 cm2/V s. With the similar triad architecture, Facchetti et al.  reported that BODIPY-quaterthiophene-BODIPY can create crystalline fiber, resulting in high electron mobility of 10−2 cm2/V s. These examples suggest that charge transport characteristic of BODIPY can be either p-type (hole mobility) or n-type (electron mobility), depending on molecular architecture (D–A–D or A–D–A). Moreover, molecular packing of BODIPY is strongly influenced on charge mobility [17, 19].
Results and discussion
Photophysical and electrochemical properties of BODIPY dyads and triads
εmax (× 104
Eg, opt (eV)
Charge transport properties
Organic thin-film transistor characteristics of BODIPY dyads and triads
μh (cm2 V−1 s−1) before annealing
μh (cm2 V−1 s−1) after annealing
V T a (V)
Ion/I off a
9.27 × 10−7
1.55 × 10−5
5.29 × 10−8
7.37 × 10−7
1.66 × 10−6
5.10 × 10−8
7.86 × 10−6
2.95 × 10−5
In summary, we have synthesized BODIPY dyads and triads using triphenylamine and carbazole as electron donating groups. The BODIPY triads demonstrated higher hole carrier mobilities, as compared to the BODIPY dyads. FET device containing CBZ-BODIPY-CBZ exhibited mobility as high as 2.95 × 10−5 cm2/V s. Although the BODIPY derivatives provided moderate performance, we hope that this work would benefit on rational design of the next BODIPY semiconductors to optimize their transistor properties.
All reagents were purchased from TCI chemicals, Aldrich and Fisher. Deuterated chloroform (CDCl3) was purchased from Cambridge Isotope Laboratories. Silica gel for column chromatography was purchased from Silicycle. All chemicals were used as received.
Synthesis of 2-iododo-BODIPY (1)
[[(3,5-Dimethyl-1H-pyrrol-2-yl)(3,5-dimethyl-2H-pyrrol-2-ylidene)methyl]methane]-(difluoroborane) or BODIPY (0.5 g, 1.9 mmol) was dissolved in chloroform (20 mL) and the reaction mixture was degassed for 10 min. A solution of N-iodosuccinimide (NIS) (0.52 g, 2.3 mmol) in anhydrous DMF (4 mL) was slowly added to a solution mixture. The reaction mixture was stirred at room temperature for 24 h. After that, the crude mixture was extracted with ethyl acetate and water. The organic layers were dried over Na2SO4 and concentrated using a rotary evaporator. The crude mixture was then purified by column chromatography over silica with DCM/hexane as an eluent to give the product 1 as orange powder (0.6 g, 80%) 1H-NMR (500 MHz, CDCl3): δ (ppm), 6.10 (s, 1H, CHPYR), 2.59 (s, 3H, CH3), 2.54–2.52 (d, 6H, CH3), 2.40–2.38 (d, 6H, CH3) MALDI-TOF MS (m/z) calculated for C14H16BF2IN2 [M + Na], 388.0073, found 411.1373.
Synthesis of 2,6-diiodo-BODIPY (2)
BODIPY precursor (0.35 g, 1.3 mmol) was dissolved in chloroform (20 mL) and the reaction mixture was degassed for 10 min. A solution of NIS (0.74 g, 3.3 mmol) in anhydrous DMF (5 mL) was slowly added to a solution mixture. The reaction mixture was stirred at room temperature for 2 days. After that, the crude mixture was extracted with ethyl acetate and water. The organic layers were dried over Na2SO4 and concentrated using a rotary evaporator. The crude mixture was then purified by column chromatography over silica with DCM/hexane as an eluent to give the product 2 as red powder (0.50 g, 74%). 1H-NMR (500 MHz, CDCl3): δ (ppm), 2.62 (s, 6H, CH3), 2.53 (s, 3H, CH3), 2.46 (d, 6H, CH3) MALDI-TOF MS (m/z) calculated for C14H15BF2I2N2, 513.9386, found 513.9382.
Synthesis of TPA-BODIPY (3)
2-Iodo-BODIPY (0.37 g, 0.94 mmol) and triphenylamine-4-boronic acid (0.35 g, 1.2 mmol) were dissolved in toluene (15 mL). The mixture was degassed for 10 min. Then, Pd(PPh3)4 (0.1 g, 0.09 mmol) and K2CO3 (2 M) were added. The reaction mixture was stirred under reflux under N2 atmosphere for 24 h. The reaction mixture was extracted with DCM and the organic layer was washed with water twice and dried over Na2SO4. The organic solvent was evaporated to dryness under reduced pressure. The residue was purified by column chromatography using DCM/hexane as the eluents to give 3 as an orange solid (0.3 g, 62%). 1H-NMR (500 MHz, CDCl3): δ (ppm), 7.29–7.26 (m, 4H, CHAR), 7.17–7.11 (m, 6H, CHAR), 7.07–7.03 (m, 4H, CHAR) 6.07 (s, 1H, CHPYR), 2.64 (s, 3H, CH3), 2.55–2.52 (d, 6H, CH3), 2.43 (s, 3H, CH3), 2.36 (s, 3H, CH3) 13C-NMR (125 MHz, CDCl3): δ (ppm), 153.49, 152.54, 147.65, 146.77, 141.37, 140.77, 137.03 133.22, 131.05, 129.29, 127.42, 1124.55, 123.05, 123.02, 121.24, 17.37, 16.58, 15.50, 14.12, 13.34 MALDI-TOF MS (m/z) calculated for C32H30BF2NO3 [M + Na], 528.2501, found 528.2523.
Synthesis of CBZ-BODIPY (4)
2-Iodo-BODIPY (0.13 g, 0.34 mmol) and 9-ethyl-carbazole-3-boronic acid (0.12 g, 0.51 mmol) were dissolve in toluene (15 mL) and degassed for 10 min. Then Pd(PPh3)4 (0.04 g, 0.003 mmol) and K2CO3 (2 M) were added. The reaction mixture was refluxed and stirred under N2 for 24 h. The reaction mixture was extracted with DCM and the organic layer was washed with water twice and dried over Na2SO4. The organic solvent was evaporated to dryness under reduced pressure. The residue was purified by column chromatography using DCM/hexane as the eluents to yield the product 4 as an orange powder (0.10, 66%). 1H-NMR (500 MHz, CDCl3): δ (ppm), 8.12 (d, J = 7.5 Hz, 1H, CHAR), 7.94 (s, 1H, CHAR), 7.51–7.44 (m, 4H, CHAR), 7.31 (m, 1H, CHAR) 7.27 (m, 1H, CHAR), 6.09 (s, 1H, CHPYR), 4.44 (q, 2H, CH2) 2.65 (s, 3H, CH3), 2.58–2.56 (d, 6H, CH3), 2.44 (s, 3H, CH3), 2.37 (s, 3H, CH3), 1.48 (t, 3H, CH3) 13C-NMR (125 MHz, CDCl3): δ (ppm), 153.14, 141.33, 140.46, 140.21, 139.08, 137.46, 134.54, 132.19, 132.00, 128.01, 125.83, 124.03, 123.00, 122.70, 122.09. 121.04, 120.40, 118.93, 108.54, 37.59, 17.32, 16.72, 15.51, 14.41, 13.84, 13.37 MALDI-TOF MS (m/z) calculated for C28H28BF2N3 [M + Na], 455.2344; found 478.224.
Synthesis of TPA-BODIPY-TPA (5)
2,6-Diiodo-BODIPY (2) (0.20 g, 0.39 mmol) and triphenylamine-4-boronic acid (0.33 g, 1.1 mmol) were dissolved in toluene (15 mL). The solution was purged with nitrogen gas for 10 min. Then, Pd(PPh3)4 (0.07 g, 0.06 mmol) and K2CO3 (2 M) were added. The reaction mixture was stirred and refluxed for 48 h. After completion of the reaction, the mixture was cooled to room temperature, followed by extraction with DCM and water. The organic layers were dried over Na2SO4 and concentrated in vacuo. The crude mixture was then purified by column chromatography using DCM/hexane as the eluents to yield 5 (0.17 g, 58%) purple solid. 1H-NMR (500 MHz, CDCl3): δ (ppm), 7.30–7.27 (m, 8H, CHAR), 7.17–7.12 (m, 12H, CHAR) 7.09–7.03 (m, 8H, CHAR) 2.72 (s, 3H, CH3), 2.55 (s, 6H, CH3), 2.37 (s, 6H, CH3)13C-NMR (125 MHz, CDCl3): δ (ppm), 152.47, 147.67,146.79, 141.29, 136.74, 133.25, 132,25, 131.09, 129.30, 127.47, 124.57, 123.07, 122.03, 17.27, 15.61, 13.38 HRMS (MALDI-TOF MS) (m/z) calculated for C50H43BF2N4, 748.3549; found 748.3662.
Synthesis of CBZ-BODIPY-CBZ (6)
2,6-Diiodo-BODIPY (2) (0.20 g, 0.4 mmol) and 9-ethylcarbazole-3-boronic acid (0.26 g, 1 mmol) were dissolved in toluene (15 mL). The solution was degassed for 10 min. Next, Pd(PPh3)4 (0.07 g, 0.06 mmol) and K2CO3 (2 M) were added. After which, the reaction mixture was stirred under reflux for 48 h. The crude mixture was extracted with EtOAc and water. The organic layers were dried over Na2SO4 and concentrated under reduced pressure. The crude product was then purified by column chromatography using DCM/hexane as the eluents to provide 6 as maroon solid (0.16 g, 64%). 1H-NMR (500 MHz, CDCl3): δ (ppm), 8.16 (d, J = 7.5 Hz, 1H, CHAR), 8.00 (s, 1H, CHAR), 7.55–7.47 (m, 6H, CHAR), 7.38 d, 2H, CHAR) 7.36–7.27 (m, 2H, CHAR), 4.47 (q, 4H, CH2) 2.79 (s, 3H, CH3), 2.61 (d, 6H, CH3), 2.45 (s, 6H, CH3), 1.54 (t, 3H, CH3) 13C-NMR (125 MHz, CDCl3): δ (ppm), 152.68, 141.21, 140.25, 139.11, 137.02 134.41, 132.21, 132.21, 128.11, 125.84, 124.22, 123.03, 122.77, 122.18. 121.45, 118.95, 108.56, 108.30, 37.65, 17.22, 15.63, 13.88, 13.41 HRMS (MALDI-TOF MS) (m/z) calculated for C42H39BF2N4 [M + Na], 671.3236; found. 671.4772.
1H-NMR spectra were recorded on 400 and 500 MHz Bruker NMR spectrometer and were reported in ppm using the solvents as the internal standard (CDCl3 at 7.26 ppm). 13C-NMR spectra were proton decoupled and recorded on a 100 MHz Bruker spectrometer using the carbon signal of the deuterated solvent as the internal standard. The exact mass measurements were recorded on Bruker Daltonics micrOTOF mass spectrometer. UV–vis absorption spectra were recorded on a Thermo Scientific UV-Genesys 10 s spectrophotometer. Electrochemical measurements were performed on a BASi Epsilon potentiostat. Charge carrier mobility was determined in field effect transistor (FET) mode using Agilent 4165C precision semiconductor parameter analyzer. Surface morphology of BODIPY films were characterized using a digital instrument dimension 3100 atomic force microscope (AFM). The cantilever specifications are described as follows: spring constant of 40 N/m, resonance frequency of 300 kHz, tip radius of 8 nm.
Bottom contacted field effect transistor (FET) devices were fabricated following our previous procedure . Briefly, the BODIPYs (3–6), concentration of 10 mg/mL in dichlorobenzene, were deposited on the FET substrate using a spin coater. All mobility measurements were performed in a glove box under argon atmosphere at a temperature of 25 °C. Thermal annealing was done in glove box under argon atmosphere. Briefly, a hot plate was pre-heated to 80 °C. Then, FET devices were placed on the hot plate and they were heated directly at the constant temperature for 3 h. After that, the devices were removed from the hot plate and allowed to cool down to room temperature and the mobilities were measured again.
SW designed and synthesized all BODIPY derivatives and characterized chemical structures using 1H-NMR, 13C-NMR and MS, performed UV–Vis spectrometry and OFET experiment and collect data. PK performed cyclic voltammetry, OFET, and AFM measurements and collect data. SW, PK and ST analyze data and wrote paper. All authors read and approved the final manuscript.
This study was funded by Thailand Research Fund (Grant no. TRG5880211) to Dr. Sompit Wanwong. Partial funding provided by the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, through its program of Center of Excellence Network Thailand is acknowledged. This work was also partially supported by the US Army Research Office (W911NF-15-1-0568) to Prof. Sankaran Thayumanavan.
The authors declare that they have no competing interests.
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